Molecular sieve layers and processes for their manufacture

ABSTRACT

Molecular sieve layers on a support are deposited from a synthesis solution while increasing the solution temperature.

This invention relates to molecular sieves, more especially tocrystalline molecular sieves, and to layers containing them. Moreespecially, the invention relates to a layer, especially a supportedlayer, containing particles of a crystalline molecular sieve, and aprocess for its manufacture.

Molecular sieves find many uses in physical, physico-chemical, andchemical processes, most notably as selective sorbents, effectingseparation of components in mixtures, and as catalysts. In theseapplications, the crystallographically-defined pore structure within themolecular sieve material is normally required to be open, and it is thena prerequisite that any structure-directing agent, or template, that hasbeen employed in the manufacture of the molecular sieve be removed,usually by calcination.

Numerous materials are known to act as molecular sieves, among whichzeolites form a well-known class. Examples of zeolites and othermaterials suitable for use in the invention will be given below.

When molecular sieves are used as sorbents or catalysts they are oftenin granular form. Such granules may be composed entirely of themolecular sieve or be a composite of a binder or support and themolecular sieve, with the latter distributed throughout the entirevolume of the granule. In any event, the granule usually contains anon-molecular sieve pore structure which improves mass transfer throughthe granule.

The support may be continuous, e.g., in the form of a plate, or it maybe discontinuous, e.g., in the form of granules. The molecular sievecrystals may be of such a size that, although the pores of the supportare occupied by the crystals, the pores remain open. Alternatively, themolecular sieve may occupy the pores to an extent that the pores areeffectively closed; in this case, when the support is continuous amolecular sieve membrane may result.

Thus, depending on the arrangement chosen and the nature and size of thematerial to be contacted by the molecular sieve, material may passthrough the bulk of the molecular sieve material entirely through thepores of the molecular sieve material, or entirely through intersticesbetween individual particles of the molecular sieve material, or partlythrough the pores and partly through the interstices.

Molecular sieve layers having the permeation path entirely through themolecular sieve crystals have been proposed for a variety of size andshape selective separations. Membranes containing molecular sievecrystals have also been proposed as catalysts having the advantage thatthey may perform catalysis and separation simultaneously if desired.

In our earlier European Patent Application No. 93.303 187.4, and acorresponding PCT Application No. PCT/E94/01301 filed simultaneouslywith this application, the disclosures of which are incorporated hereinby reference, there are described a supported inorganic molecular sievelayer having a controllable thickness that may, if desired, be of athickness of the order of only a few microns, and processes for itsmanufacture.

Such a layer and a process for its manufacture make possible theproduction of a number of useful products, including membranes, whichbecause of their uniformity and thinness will have predictableproperties, and will permit a high flux.

One process according to our earlier application comprises preparing asynthesis mixture comprising a source of silica and an organic structuredirecting agent in the form of a hydroxide in a proportion sufficient toeffect substantially complete dissolution of the silica source in themixture at the boiling temperature of the mixture, immersing the supportin the synthesis mixture, crystallizing zeolite from the synthesismixture onto the support, and if desired or required calcining thecrystallized layer.

This process gives a uniform layer which is of sufficient thickness formany purposes. For other purposes, however, a layer of greater thicknessis desirable.

The present invention provides a process for the manufacture of a layercomprising a crystalline molecular sieve on a porous support, whichcomprises preparing a synthesis mixture comprising a source of silicaand an organic structure directing agent, immersing the support in thesynthesis mixture and crystallizing zeolite from the synthesis mixtureonto the support, the temperature of the synthesis mixture beingincreased during crystallization.

The synthesis mixture will also contain a source of other components, ifany, in the zeolite.

Subsequently to crystallization, if desired or required the supportedlayer may be calcined.

Advantageously, the source of silica is in solution in the synthesismixture, and dissolution may be facilitated by employing the structuredirecting agent in a proportion sufficient to effect substantiallycomplete dissolution of the source at the boiling temperature of themixture.

The synthesis mixture is advantageously prepared as described inInternational Application WO93/08125, the disclosure of which isincorporated herein by reference.

The increase in temperature may be continuous but is advantageouslystepwise. Advantageously, the temperature is increased within the rangeof from 70° C. to 170° C., and preferably from 90° C. to 150° C., overthe crystallization period. Advantageously, the temperature at the endof the crystallization period is at least 120° C. That period may, forexample, range from 24 hours to 7 days, and is preferably between 3 and6 days. Advantageously, the temperature is increased at least once afterdeposition of crystals on the support has commenced. Preferably thetemperature is increased at least twice after deposition has commenced.

Conveniently, the temperature is increased in steps after a time longenough to bring about deposition, and advantageously substantialdeposition, at the lower temperature.

Preferably, the synthesis mixture is maintained at each temperaturestage until deposition, as measured by weight increase, has ceased atthat temperature. This time tends to decrease with increase intemperature but may be, for example, from 4 to 48 hours, advantageouslyfor from 8 to 24 hours. The increase in temperature at each step mayconveniently be at least 20° C.

The process has advantages over other methods of obtaining thickerlayers which require washing of the support after initial or anyprevious deposition and replacement of the synthesis mixture.

Advantageously, the mean particle size of the crystalline molecularsieve in the layer is within the range of from 20 to 500 nm, preferablyit is within the range of from 20 to 300 nm and most preferably withinthe range of from 20 to 200 nm. Alternatively, the mean particle size isadvantageously such that at least 5% of the unit cells of the crystalare at the crystal surface.

Advantageously, the particle size distribution is such that 95% of theparticles have a size within ±33% of the mean, preferably 95% are within±10% of the mean and most preferably 95% are within ±7.5% of the mean.

The invention also provides a supported layer made by the process of theinvention. The layer comprises molecular sieve particles; these areidentifiable as individual particles (although they may be intergrown asindicated below) by electron microscopy. The layer, at least aftercalcining, is mechanically cohesive and rigid. Within the intersticesbetween the particles in this rigid layer, there may exist non-molecularsieve pores, which may be open, or partially open, to permit passage ofmaterial through or within the layer, or may be completely sealed,permitting passage through the layer only through the pores in theparticles.

It will be understood that the particle size of the molecular sievematerial forming the layer may vary continuously or stepwise withdistance from the support. In such a case, the layer is considered to beuniform if the particle size distribution is within the defined limit atone given distance from the support, although advantageously theparticle size distribution will be within the defined limit at eachgiven distance from the support.

The use of molecular sieve crystals of small particle size andpreferably of homogeneous size distribution facilitates the manufactureof a three-dimensional structure which may if desired be thin but whichis still of controlled thickness.

Advantageously, the particles are contiguous, i.e., substantially everyparticle is in contact with one or more of its neighbours, although notnecessarily in contact with all its closest neighbours. Such contact maybe such in some embodiments that neighbouring crystal particles areintergrown, provided they retain their identity as individualcrystalline particles. Advantageously, the resulting three dimensionalstructure is grain-supported, rather than matrix-supported, if the layerdoes not consist essentially of the crystalline molecular sieveparticles. In a preferred embodiment, the particles in the layer areclosely packed.

It will be understood that references herein to the support of a layerinclude both continuous and discontinuous supports.

References to particle size are to the longest dimension of the particleand particle sizes are as measured by direct imaging with electronmicroscopy. Particle size distribution may be determined by inspectionof scanning electron micrograph images, and analysing an appropriatelysized population of particles for particle size.

As molecular sieve, there may be mentioned a silicate, analuminosilicate, an aluminophosphate, a silicoaluminophosphate, ametalloaluminophosphate, or a metalloaluminophosphosilicate.

The preferred molecular sieve will depend on the chosen application, forexample, separation, catalytic applications, and combined reactionseparation. There are many known ways to tailor the properties of themolecular sieves, for example, structure type, chemical composition,ion-exchange, and activation procedures.

Representative examples are molecular sieves/zeolites of the structuretypes AFI, AEL, BEA, CHA, EUO, FAU, FER, KFI, LTA, LTL, MAZ, MOR, MFI,MEL, MTW, OFF, TON and faujasite. The process of the invention isespecially suited to the manufacture of supported layers of MFI, MEL,and faujasite structures.

Some of the above materials while not being true zeolites are frequentlyreferred to in the literature as such, and this term is used broadly inthis specification.

Advantageously, the silica is advantageously introduced into thesynthesis mixture in particulate form e.g., as silicic acid powder.

The organic structure directing agent is advantageously introduced intothe synthesis mixture in the form of a base, specifically in the form ofa hydroxide, but a salt, e.g, a halide, especially a bromide, may beemployed.

The structure directing agent may be, for example, the hydroxide or saltof tetramethylammonium (TMA), tetraethylammonium (TEA),triethylmethylammonium (TEMA), tetrapropylammonium (TPA),tetrabutylammonium (TBA), tetrabutylphosphonium (TBP),trimethylbenzylammonium (TMBA), trimethylcetylammonium (TMCA),trimethylneo-pentylammonium (TMNA), triphenylbenzylphosphonium (TPBP),bispyrrolidinium (BP), ethylpyridinium (EP), diethylpiperidinium (DEPP)or a substituted azoniabicyclooctane, e.g. methyl or ethyl substitutedquinuclidine or 1,4-diazoniabicyclo-(2,2,2)octane.

Preferred structure directing agents are the hydroxides of TMA, TEA, TPAand TBA.

The thickness of the molecular sieve layer is advantageously within therange of 0.1 to 15 μm, preferably from 0.1 to 2 μm. Advantageously, thethickness of the layer and the particle size of the molecular sieve aresuch that the layer thickness is at least twice the particle size,resulting in a layer several particles thick rather than a monolayer ofparticles.

Advantageously, the layer is substantially free of pinholes, i.e.,substantially free from apertures of greatest dimension greater than 0.1μm. Advantageously, at most 0.1% and preferably at most 0.0001% of thesurface area is occupied by such apertures.

Depending on the intended end use of the layer, a greater or smallerproportion of the area of the layer may be occupied by macropores,apertures having a greatest dimension less than 0.1 μm but greater than1 nm. These macropores may be formed by the interstices between thecrystals of the molecular sieve, if the layer consists essentially ofthe molecular sieve, and elsewhere, if the layer comprises the molecularsieve and other components. Such layers may be used, inter alia, forultrafiltration, catalytic conversion, and separations based ondifferences in molecular mass (Knudsen diffusion), and indeed for anyprocesses in which a high surface area is important.

The layer advantageously has a large proportion of its area occupied bycrystalline-bounded micropores, i.e., pores of a size between 0.2 and 1nm, depending on the particular molecular sieve being employed. Pores ofsize within the micropore range result, for example, when the layercontains a component in addition to one derived from colloidal molecularsieve particles. In another embodiment especially suitable forultrafiltration, the layer contains nanopores, i.e., pores of a sizebetween 1 and 10 nm.

The layer support may be either non-porous or, preferably, porous, andmay be continuous or particulate. As examples of non-porous supportsthere may be mentioned glass, fused quartz, and silica, silicon, denseceramic, for example, clay, and metals. As examples of porous supports,there may be mentioned porous glass, sintered porous metals, e.g., steelor nickel (which have pore sizes typically within the range of 0.2 to 15μm), and, especially, an inorganic oxide, e.g., alpha-alumina, titania,an alumina/zirconia mixture, or Cordierite.

At the surface in contact with the layer, the support may have pores ofdimensions up to 50 times the layer thickness, but preferably the poredimensions are comparable to the layer thickness.

Advantageously, the support is porous alpha-alumina with a surface poresize within the range of from 0.08 to 10 μm, preferably from 0.08 to 1μm, most preferably from 0.08 to 0.16 μm, and advantageously with anarrow pore size distribution. The support may be multilayered; forexample, to improve the mass transfer characteristics of the layer, onlythe surface region of the support in contact with the layer may havesmall diameter pores, while the bulk of the support, toward the surfaceremote from the layer, may have large diameter pores. An example of sucha multilayer support is an alpha-alumina disk having pores of about 1 μmdiameter coated with a layer of alpha-alumina with pore size about 0.08μm.

The invention also provides a structure in which the support, especiallya continuous porous support, has a molecular sieve layer on each side ofthe support, the layers on the two sides being the same or different.

The layer may, and for many uses advantageously does, consistessentially of the molecular sieve material, or it may be a composite ofthe molecular sieve material and intercalating material which is alsoadvantageously inorganic. The intercalating material may be the materialof the support. If the layer is a composite it may, as indicated above,contain macropores and/or micropores, bounded by molecular sieveportions, by portions of intercalating material, or by both molecularsieve and intercalating material. The material may be applied to thesupport simultaneously with or after deposition of the molecular sieve,and may be applied, for example, by a sol-gel process followed bythermal curing. Suitable materials include, for example, inorganicoxides, e.g., silica, alumina, and titania.

The intercalating material is advantageously present in sufficiently lowa proportion of the total material of the layer that the molecular sievecrystals remain contiguous.

The layer produced in accordance with the processes of the invention maybe treated in manners known per se to adjust their properties, e.g., bysteaming or ion exchange to introduce different cations or anions, bychemical modification, e.g., deposition of organic compounds into thepores of the molecular sieve, or by introduction of a metal.

The layer may be used in the form of a membrane, used herein to describea barrier having separation properties, for separation of fluid(gaseous, liquid, or mixed) mixtures, for example, separation of a feedfor a reaction from a feedstock mixture, or in catalytic applications,which may if desired combine catalysed conversion of a reactant orreactants and separation of reaction products.

Separations which may be carried out using a membrane comprising a layerin accordance with the invention include, for example, separation ofnormal alkanes from co-boiling hydrocarbons, especially n-C₁₀ to C₁₆alkanes from kerosene; separation of aromatic compounds from oneanother, especially separation of C₈ aromatic isomers from each other,more especially para-xylene from a mixture of xylenes and, optionally,ethylbenzene, and separation of aromatics of different carbon numbers,for example, mixtures of benzene, toluene, and mixed C₈ aromatics;separation of aromatic compounds from aliphatic compounds, especiallyaromatic molecules with from 6 to 8 carbon atoms from C₅ to C₁₀ (naphtharange) aliphatics; separation of olefinic compounds from saturatedcompounds, especially light alkenes from alkane/alkene mixtures, moreespecially ethene from ethane and propene from propane; removinghydrogen from hydrogen-containing streams, especially from lightrefinery and petrochemical gas streams, more especially from C₂ andlighter components; and alcohols from aqueous streams.

The supported layer of the invention may be employed as a membrane insuch separations without the problem of being damaged by contact withthe materials to be separated. Furthermore, many of these separationsare carried out at elevated temperatures, as high as 500° C., and it isan advantage of the supported layer of the present invention that it maybe used at such elevated temperatures.

The present invention accordingly also provides a process for theseparation of a fluid mixture which comprises contacting the mixturewith one face of a layer according to the invention in the form of amembrane under conditions such that at least one component of themixture has a different steady state permeability through the layer fromthat of another component and recovering a component or mixture ofcomponents from the other face of the layer.

The invention further provides a process for catalysing a chemicalreaction which comprises contacting a feedstock with a layer accordingto the invention which is in active catalytic form under catalyticconversion conditions and recovering a composition comprising at leastone conversion product.

The invention further provides a process for catalysing a chemicalreaction which comprises contacting a feedstock with one face of a layeraccording to the invention, that is in the form of a membrane and inactive catalytic form, under catalytic conversion conditions, andrecovering from an opposite face of the layer at least one conversionproduct, advantageously in a concentration differing from itsequilibrium concentration in the reaction mixture.

The following examples illustrate the invention:

EXAMPLE 1

A porous alpha-alumina disk with a pore diameter of 160 nm, polished onone side, was cut into four equal-sized parts. The parts were weighedand placed, polished side up, on ptfe rings resting on the bottom of astainless steel autoclave. In the autoclave was poured 70.22 g of asynthesis solution with a molar composition of

    10 SiO.sub.2 /1.56 (TPA).sub.2 0/0.275 Na.sub.2 0/147 H.sub.2 0

The open autoclave was placed in an exsiccator, which was then evacuatedover 0.5 hours to increase the penetration of synthesis solution intothe disk pieces. The autoclave was taken out of the exsiccator, closed,and placed in an oven at room temperature. The oven was heated up to 90°C. over a few minutes and kept at that temperature for 48 hours. Theautoclave was then cooled to room temperature, opened and one of thesupport pieces was removed. The autoclave was closed again and placed inan oven at room temperature. The oven was heated up to 110° C. over afew minutes and kept at that temperature for 24 hours. The autoclave wascooled down again and a second piece was removed. The temperature cyclewas repeated twice, first for 24 hours at 130° C. and then for 24 hoursat 150° C. The four pieces of the disk were all washed withdemineralized water at 70° C. until the washing water had a conductivityof about 6 micro Siemens/cm, dried at 105° C. and cooled to roomtemperature in an exsiccator. It was observed that with each ageing stepthe weight of the disk pieces increased, as shown in the followingtable:

    ______________________________________                                        Disk Piece #                                                                             Temperature History,°C.                                                               Weight Increase,%                                   ______________________________________                                        1          90             0.88                                                2          90; 110        2.04                                                3          90; 110; 130   3.50                                                4          90; 110; 130; 150                                                                            5.63                                                ______________________________________                                    

X-ray diffraction showed that with each ageing step the intensity of thezeolite peaks increased with respect to the intensity of thealpha-alumina peaks, as shown in the following table:

    ______________________________________                                                   peak intensity ratio:                                                         peak at d = 0.385 nm (MFI)/                                        disk piece peak at d = 0.348 nm (Al203)                                       ______________________________________                                        1          0.190                                                              2          0.217                                                              3          0.236                                                              4          0.332                                                              ______________________________________                                    

These results indicate that with each ageing step at a highertemperature new zeolite crystals are deposited on the support.

Example 2

In a similar experiment, the procedure of Example 1 was repeated, butwithout cooling the autoclave to remove disk pieces. A disk similar tothat of Disk No. 4 of Example 1 resulted, with a weight increase of5.38%.

I claim:
 1. A process for the manufacture of a layer comprising acrystalline molecular sieve on a support comprising:a) preparing asynthesis mixture comprising a source of silica and an organicstructure-directing agent; b) immersing said support in said synthesismixture; c) heating said synthesis mixture containing said immersedsupport to an initial temperature of at least 70° C. and maintainingsaid synthesis mixture at said initial temperature for a period ofcrystallization time sufficient to deposit zeolite crystals onto saidsupport; and d) further heating said synthesis mixture containing saidimmersed support to a second temperature higher than said initialtemperature for a period of crystallization time sufficient to depositadditional zeolite crystals onto said support and to form said layer. 2.The process of claim 1 wherein said crystals in said layer have aparticle size in the range of from 20 to 500 nm.
 3. The process of claim1 wherein said crystals in said layer have a particle size of in therange of from 20 to 300 nm.
 4. The process of claim 1 wherein said layercontains nanopores having a dimension of 1 to 10 nm.
 5. The process ofclaim 1 wherein said layer contains micropores having a dimension of 0.2to 1 nm.
 6. The process of claim 1 wherein said synthesis mixture ismaintained at said initial temperature until deposition of zeolitecrystals at said temperature has ceased.
 7. The process of claim 1 whichis a stepwise process and wherein said synthesis mixture is maintainedat said second temperature for a period of crystallization timesufficient to deposit additional zeolite crystals onto said support. 8.The process of claim 7 wherein said synthesis mixture containing saidimmersed support is further stepwise heated to at least one additionaltemperature higher than said second temperature and wherein saidsynthesis mixture is maintained at said at least one additionaltemperature for a period of crystallization time sufficient to depositadditional zeolite crystals onto said support.
 9. The process of claim 8wherein the maximum additional heating temperature is up to 170° C. 10.The process of claim 8 wherein the maximum additional heatingtemperature ranges from 120° to 170° C.
 11. The process of claim 1wherein said synthesis mixture is heated in step (d) to a maximumtemperature of up to 170° C.
 12. The process of claim 1 wherein saidsynthesis mixture is heated in step (d) to a minimum temperature of atleast 120° C.
 13. The process of claim 1 wherein said support is porous.14. The process of claim 13 wherein said layer has a thickness in therange of 0.1 to 15 μm.
 15. A process as claimed in claim 1, wherein thecrystallization period is from 24 hours to 7 days.
 16. A process asclaimed in claim 15, wherein the period is from 3 to 6 days.
 17. Aprocess as claimed in claim 1, wherein after its deposition on thesupport the supported zeolite layer is calcined.
 18. A process for themanufacture of a layer comprising a crystalline molecular sieve on aporous support comprising:a) preparing a synthesis mixture comprising anaqueous solution of a source of silica and an organicstructure-directing agent; b) immersing said support in said synthesismixture; c) heating said synthesis mixture containing said immersedsupport in a first step to a first temperature of at least 70° C. andmaintaining said synthesis mixture at said first temperature for aperiod of crystallization time sufficient to deposit zeolite crystalsonto said support; d) further heating said synthesis mixture containingsaid immersed support from step (c) in a second step to a secondtemperature higher than said first temperature and maintaining saidsynthesis mixture at said second temperature for a period ofcrystallization time sufficient to deposit additional zeolite crystalsonto said support; and e) further heating said synthesis mixturecontaining said immersed support from step (d) in at least oneadditional step to at least one additional temperature higher than saidsecond temperature and maintaining said synthesis mixture at each ofsaid additional temperatures for a period of crystallization timesufficient to deposit additional zeolite crystals onto said support andto form said layer, the maximum temperature to which said synthesismixture is heated ranging from 120° to 170° C.
 19. The process of claim18 wherein the temperature increase in each heating step is at least 20°C.